This invention relates to corona ionisation sources and in particular to continuous corona ionisation sources used in ion mobility spectrometry.
Ion mobility spectrometers are used in numerous applications such as the detection of narcotics, explosives and chemical warfare agents in air and for environmental monitoring.
A typical ion mobility spectrometer (IMS) such as that described in U.S. Pat. No. 4,777,363 comprises an ionisation source, a reaction region and a drift region. As monitoring/detection takes place at ambient atmospheric pressure, ionisation of the sample gas at atmospheric pressure is required. After ionisation, ions generated from the sample gas are expelled into a drift region where, under the influence of an electric field and collisions with a counter-flowing drift gas, the ions attain a constant velocity before arriving at a collector. The ion mobility spectrum obtained is characteristic of the sample being investigated.
Radioactive materials, such as 63Ni, are traditionally used as the ionisation sources in ion mobility spectrometers. The output from such radioactive sources is highly stable, and, in addition, they are noise and power free. However, radioactive sources have to be handled and disposed of with great care and the exposure of operating personnel to ionising radiation has to be carefully controlled and monitored. Taking all the necessary precautions in relation to the use, transportation, storage and disposal of devices incorporating radioactive sources can therefore prove costly.
Continuous corona ionisation sources have previously been used in IMS systems as an alternative to the use of radioactive sources. The ions produced by an IMS incorporating a continuous corona ionisation source and operating in negative mode (i.e. producing and collecting negatively charged ions) have however been found to differ markedly from the ions produced using a radioactive 63Ni ionisation source. See, for example, B Gravendee and F J de Hoog, J Phys B: At Mol Phys 20, 6337 (1987) for a discussion of the ionic species produced when air is ionised by a continuous corona ionisation source. The major problem is that neutral species formed during the ionisation process react with the initially formed reactant ion species. This results in the formation of unreactive ions which are significantly more stable to reaction than those produced by radioactive 63Ni ionisation sources, and which do not react so readily with sample vapour. Consequently, the sensitivity of IMS systems incorporating continuous corona ionisation sources can be low and it is commonly accepted by those skilled in the IMS field that continuous corona ionisation sources are unsuitable alternatives to traditional radioactive ionisation sources.
Various pulsed corona discharge sources have also been developed and, to a limited extent, these overcome some of the disadvantages associated with the use of continuous corona ionisation sources that are described above. However, pulsed corona discharge sources are expensive, involve complex pulsing, triggering and timing delays and have to be synchronised with gate opening events. As such there is still a need for improvements to the corona ionisation apparatus to improve ion mobility spectral data to make this a viable alternative to radioactive ionisation sources. Furthermore, there remains a need to develop a corona discharge ionisation apparatus, for use with an ion mobility spectrometer, whereby the unwanted side reaction between the neutral species and the reactant ion species, both formed during the initial ionisation stage, is minimised or eliminated. This would ensure that a more efficient reaction is achieved between the initially formed reactant ion species and the sample material, in turn improving the spectral results.
Various ion mobility spectrometers have been disclosed in the prior art. For example U.S. Pat. No. 4,445,038 discloses an apparatus for simultaneous detection of positive and negative ions in ion mobility spectrometry comprising dual drift regions respectively on either side of a centrally located reaction region; U.S. Pat. No. 5,234,838 discloses an ion mobility spectrometer for the analysis of ammonia whereby dimethly methyl phosphonate is added to the carrier gas stream prior to application of the carrier gas stream into the ionisation chamber thus forming clusters with the ammonia which have different drift times; and U.S. Pat. No. 5,283,199 discloses an ion mobility spectrometer for the analysis of chlorine dioxide whereby a controlled quantity of amine is added to the carrier gas stream prior to application of the carrier gas stream into the ionisation chamber thus suppressing the chlorine peak.
Although these documents optionally disclose the use of corona ionisation sources none of the documents specifically address the problem of improvement of the quality of corona ionised ion mobility spectra. More specifically they do not address the problem of how to minimise, or eliminate, the interaction of the neutral and reactant ion species in the ionised gas. Interestingly the apparatus of each of these documents has a configuration such that a certain flow of drift gas, whose primary role is to separate ions in the drift region, may pass from the drift region into the ionisation region of the spectrometer. However, this has the secondary effect of interfering with the flow of material in the ionisation region including sweeping neutral material to the exhaust. Such neutral material may include sample material introduced into the ionisation region and neutral species produced during the ionisation process.
Several problems are associated with configurations such as those described in the prior art. These include that the drift gas may not pass sufficiently closely to the ionisation source to efficiently remove the problematic neutral species formed in the ionisation gas; that the rate of flow of the drift gas into the ionisation region may be inconsistent as a result of having to pass through a shutter grid; that it is not possible to adjust the flow of any drift gas in the ionisation region separately from the flow rate in the drift region; and that the removal of neutral material will result in dilution of the sample material with a corresponding reduction in sensitivity.
Thus there remains the problem of how to efficiently remove the neutral species formed during the ionisation process from the ionised gas thus minimising or eliminating the interaction of the neutral and initially formed reactant ion species. There also remains the problem of how to maximise the efficiency of the reaction between the initially formed reactant ion species and the sample material. Finally there remains the problem of how to achieve this effect without affecting the use of the drift gas to separate ions in the drift region of the spectrometer. It is an object of this invention to mitigate some of the disadvantages, as described above, that are associated with the use of corona ionisation sources.
According to a first aspect the present invention relates to an apparatus for ionisation of a gas comprising a corona ionisation source, a means for flowing gas past the corona ionisation source, and a means for applying an electric field to move any ions produced by the corona ionisation source away from the corona ionisation source, characterised in that the direction of gas flow through the corona ionisation source is substantially different to the direction of the ions in the esectric field.
According to a second aspect, the present invention relates to an ion mobility spectrometer comprising an apparatus for ionisation of a gas as characterised in the first aspect of the invention.
Herein, the term xe2x80x9csubstantially differentxe2x80x9d shall be taken to mean a difference between the direction of electric field induced ion flow from the corona ionisation source and the direction of gas flow past the corona ionisation source, such that ions produced by the ionisation source become spatially separated from any neutral species produced by the ionisation source.
Herein the term xe2x80x9creactant ionsxe2x80x9d relates to the reactive ions which are initially formed when the gas is ionised by the corona discharge source. Ideally these ions later react with the sample material to form ionised sample species which then pass into the drift region and are separated prior to detection by the ion mobility spectrometer.
Herein the term xe2x80x9cneutral speciesxe2x80x9d relates to the neutral species which are initially formed when the gas is ionised by the corona discharge source.
Herein the term xe2x80x9cunreactive ionsxe2x80x9d relates to the ions which form as a result of the side reaction which occurs between the reactant ion species and the neutral species. These unreactive ions are not able to further react with the sample material.
Directing the flow of ions and the flow of gas from the ionisation source in substantially different directions, and thus spatially separating ions and neutral species formed by the corona, prevents any further chemical reactions between the neutral species and the reactant ions from occurring. This provides ion mobility spectra substantially similar to those produced using radioactive ionisation sources, and mitigates some of the disadvantages of using corona ionisation sources in ion mobility spectrometers that are described above. Any inert gas would be useful in the present invention to be used to flow past the corona ionisation source in a substantially different direction to the flow of ions in the electric field. Optionally the inert gas may be dried before use to remove some or all of the water that may be present. It is preferred that the inert gas is air.
Advantageously, the corona ionisation source is a continuous corona ionisation source. Continuous corona ionisation sources have the advantage, compared to pulsed corona discharge ionisation sources, of being relatively inexpensive. Continuous corona ionisation sources are also free from the complex pulsing, triggering, delay timing and shutter synchronisation requirements of pulsed corona sources. Many different corona ionisation sources are available and the exact source to be used will vary depending on the source material, the corona energy and the inert gas to be ionised. One of ordinary skill in the art will be able to identify a suitable corona ionisation source for use in any given instance. The corona ionisation sources comprise a needle which can be made from a wide variety of metal materials. Commonly used examples include gold, platinum, steel, stainless steel and many metal alloys. The diameter of the corona needle will vary depending on the conditions, particularly the corona energy which is being applied. Examples of needle sizes include a 10 xcexcm diameter, a 50 xcexcm diameter, or a large atmospheric pressure chemical ionisation needle which has a diameter of 10,000 xcexcm tapering down to a point. In order to supply the corona energy required for ionisation of the inert gas a current is applied to the needle. A wide range of currents can be used depending on the diameter of the needle and the inert gas to be ionised. Again one of ordinary skill in the art will be able to identify most suitable current required for the specific circumstances by routine experiment.
Conveniently, the flow of gas past the corona ionisation source may be continuous or alternatively the flow may be periodic. In other words, the gas may either flow continuously past the corona ionisation source or it may be periodically flushed past the corona ionisation source. It is preferred that if a continuous corona ionisation source is used that the flow of gas past the source in a direction substantially different to the direction of flow of the ions in the electric field is also continuous. A range of air flow rates can be used. Again the exact flow rate of the gas past the corona needle will vary from system to system depending on the corona needle size, the corona energy and the inert gas to be ionised and will therefore need to be optimised by one skilled in the art by routine experimentation. It is likely that the lower the corona energy the lower the flow rate that will be required. It is desirable that the flow rate of gas be adjustable such that it can be varied until the optimum rate is determined for the system in question. In an optimum operation the flow rate should be adjusted such that it is able to adequately flush neutral species from the ionised gas without affecting the flow of ionised species away from the corona needle in the electric field. This can be determined by routine experiment by one skilled in the art by optimising the sensitivity of the system to either maximise the concentration of initially formed reactant ions or by maximising the concentration of the sample product ion.
In order to minimise complexity of the ionisation chamber it is preferred that the inert gas to be ionised and the inert gas that flows past the corona ionisation source in a direction substantially different to the direction of flow of the ions in the electric field is the same gas. In this mode some of the inert gas will be ionised by the corona ionisation source as it flows past the source and other parts of the gas will be used to flush away any neutral species form in the ionised gas.
The sample material can be introduced either into the ionisation region such that the sample gas mixes with the ionised gas prior to the mixture passing into a reaction region or the sample material can be introduced directly into the reaction region. It is preferred that the flow rate of the gas past the ionisation source be adjusted such that it is able to flush neutral species from the ionised inert gas whilst flushing as little of the sample material from the ionisation region or reaction region as possible. One advantage of introducing the sample directly into the reaction region is that the flow of inert gas past the ionisation source can be isolated from the introduction of the sample material. This means that the sample material will not be diluted by a flow of gas and the reaction between the sample material and the ionised species will be concentrated thus improving its efficiency.
It is highly preferred that the apparatus also comprises a means for flowing gas through the drift region which is exhausted from either the reaction region or the drift region. It is preferred that this exhaust point is arranged within the apparatus such that it does not interfere with the flow of either the sample material or any ionised material in the apparatus. Such an apparatus comprises at least two means for flowing gasxe2x80x94a first means which flows gas past the ionisation source and the second means which flows gas through the drift region. Furthermore, it is preferred that each of the means is independently adjustable such that the flow rate of each gas can be separately altered. This allows that the flow rate of the gas in the ionisation chamber can be optimised to efficiently flush out neutral species. Independently the flow rate of the gas through the drift region can be optimised to give good separation of the ionised sample material. It is likely that, in order to achieve optimal spectral results, that these two flow rates will be different. In order to maximise the benefit from such a design it is preferred that the apparatus has a drift gas inlet and a drift gas outlet in the drift region allowing for throughput of the drift gas. The apparatus should have a further flush inlet either in the ionisation region, or in the reaction region, with a flush outlet in the ionisation region to allow for throughput of the flush gas past the ionisation source in a direction substantially different to the direction of flow of the ions in the electric field. The flush outlet should be positioned in the ionisation region so as to achieve optimum flow of the gas past the ionisation source.
Although the drift region can comprise any drift tube, in an apparatus of the present invention it is preferred that the drift tube is a flexible modular system constructed of alternating rings of gold plated stainless steel and ceramic such as those that can be supplied by Graseby Ionics (Watford, UK). It is preferred that ions are gated into the drift region using a suitable gating device such as a Bradbury Neilsen shutter. The drift tube may be assembled to be any convenient length and again this will be determined by one skilled in the art on a case by case basis by routine experimentation and depending on the sensitivity and resolution required. Finally any suitable field strength can be applied to the drift tube and one of ordinary skill in the art will be able to identify a suitable field.
In a further embodiment the apparatus for ionisation of a gas comprises an ionisation region comprising a corona ionisation source, a reaction region and a drift region, wherein the means for flowing gas past the corona ionisation source has a flush inlet in the ionisation region and a flush outlet also in the ionisation region such that the direction of gas flow through the corona ionisation source is substantially different to the direction of flow of the ions in the electric field, and wherein the sample material is introduced into the reaction region. In this arrangement the neutral species produced by the corona source are flushed from the ionisation region whereas the reactant ions pass into the reaction region to react with the sample. This apparatus has several advantages. Firstly the same inert gas can readily be used as the ionisation gas and flush gas. Secondly the flow rate can be adjusted to maximise the flush rate of neutral species to minimised unwanted side reactions without having to be additionally concerned with inadvertently flushing away any unreacted sample material. Finally the neutral species are fully flushed before the reactant ions interact with the sample material such that there is a reduced possibility of side reactions which degrade the spectra. The apparatus can either be operated in the negative mode whereby negatively charged ions are detected, or in the positive mode whereby positively charged ions are detected. It is also possible that the apparatus could be arranged for simultaneous detection of positive and negative ions. This invention produces particular improvement to results when the apparatus is operated in the negative mode.
Ideally the gas contains air, and it may also comprise a sample of chemical vapour such as vapour from one or more of the following chemical warfare agents, explosives, narcotics or atmospheric pollutants and the like.
In another preferred embodiment, an ion mobility spectrometer comprises the apparatus for ionisation of a gas that is described above. The spectral results achieved from such a machine can be enhanced if Fourier transform techniques are applied to such an ion mobility spectrometer. This has the potential to enhance both the sensitivity and resolution of the signal compared to that achieved by a standard instrument.